1. Field of the Invention
The present invention relates to a trigger source switching circuit for selecting a trigger source signal, which is applied to an oscilloscope or the like.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a trigger source switching circuit used in a conventional oscilloscope. In FIG. 1, a switching circuit 21 for selecting one of four trigger signals S1-S4 is connected in cascade with a coupling circuit 22. The coupling circuit 22 comprises a capacitor C3, a switch SW8 connected in parallel with the capacitor C3, a resistor R11, and a switch SW9 connected in series with the capacitor C3. The capacitor C3 is for passing an alternating current signal, and the switch SW8 is for passing a direct current signal. The switch SW9 is for selecting one of the following three modes: a through mode; a high rejection mode by using a low-pass filter composed of a resistor 12 and a capacitor C4, and a low rejection mode by using a high pass filter composed of a capacitor C5 and a resistor R13.
As a switching circuit 21, four semiconductor switches, each of which has an arrangement as shown in FIG. 2, are used to be controlled by a CPU or the like. Each semiconductor switch comprises two stage emitter followers which are composed of a transistor Q7 and a resistor R14 and a transistor Q8 and a resistor R15, respectively, and a switch SW10 for supplying a voltage Vcc to the base of the transistor Q8.
The circuit of FIG. 1, however, includes at least two switches in the passage of the trigger signal, and hence, deteriorates the high frequency characteristic.
Furthermore, although the semiconductor switch shown in FIG. 2 has good frequency and isolation characteristics, it has variations and temperature drift in the DC component of the output due to variations and temperature drift in the base-to-emitter voltage V.sub.BE of the transistors Q7 and Q8.